1) Field of the Invention
The invention is in the field of Microelectromechanical Systems (MEMS).
2) Description of Related Art
For the past several years, MEMS structures have been playing an increasingly important role in consumer products. For example, MEMS devices, such as sensors, detectors and mirrors, can be found in products ranging from air-bag triggers in vehicles to displays in the visual arts industry. As these technologies mature, the demands on precision and functionality of the MEMS structures have escalated. For example, optimal performance may depend on the ability to fine-tune the characteristics of various components of these MEMS structures. Furthermore, consistency requirements for the performance of MEMS devices (both intra-device and device-to-device) often dictate that the processes used to fabricate such MEMS devices need to be extremely sophisticated.
A recent fabrication challenge in the field MEMS devices is the formation of MEMS resonators. A typical MEMS resonator has at least a portion of the MEMS structure “suspended” above a substrate, i.e. not directly attached to the substrate, thus requiring the incorporation of a release layer at some point in the fabrication process. A damascene approach utilizing a sacrificial damascene layer has been employed to fabricate such resonators. This method, however, has its limitations.
A damascene approach incorporating a sacrificial damascene layer as a release layer has been used to fabricate a MEMS resonator. FIGS. 1A-C illustrate cross-sectional views representing a series of steps in a damascene approach for fabricating a MEMS structure having a member suspended above a substrate, in accordance with the prior art.
Referring to FIG. 1A, a trench 106 consisting of a first feature 108 and a second feature 110, usually patterned in two separate steps, is patterned into release layer 104 above a substrate 102. Trench 106 is then filled with a structural layer 112, usually by blanket deposition of a material layer followed by planarization of the material layer, as depicted in FIG. 1B. Referring to FIG. 1C, release layer 104 is removed to provide a MEMS resonator 114 comprised of a resonating member 116 (formed from second feature 110 of trench 106) connected to substrate 102 by an anchor 118 (formed from first feature 108 of trench 106). One drawback to this approach is that the second feature 110 is patterned into release layer 104 by a timed etch process in order to partially etch release layer 104. Thus, variability in the etch process used will lead to variability of the dimensions of the MEMS resonator 114.
A damascene approach incorporating a sacrificial damascene layer as a release layer has also been used to fabricate a MEMS resonator flanked by a pair of electrodes. FIGS. 2A-D illustrate cross-sectional views representing a series of steps in a damascene approach for fabricating a MEMS structure having a member suspended above a substrate and between a pair of electrodes, in accordance with the prior art.
Referring to FIG. 2A, a mask 220 comprised of blade portions 222 is formed above a release layer 204, which is above a substrate 202. Trenches 206 consisting of first features 208 and second features 210, usually patterned in two separate steps, are patterned into release layer 204, as depicted in FIG. 2B. Referring to FIG. 2C, trenches 206 are then filled with a structural layer 212, usually by blanket deposition of a material layer followed by planarization of the material layer. Release layer 204 is then removed to provide a MEMS resonator 214 comprised of a resonating member 216 and flanked by a pair of electrodes 224, all of which are connected to substrate 202 by anchors 218 (formed from first features 208 of trenches 206), as depicted in FIG. 2D. One drawback to this approach is that blade portions 222 from mask 220 are used to ultimately define the spacing between a resonating member 216 and a pair of electrodes 224. Thus, the spacing is confined to lithographic constraints and the surface roughness of the sidewalls of the resonating member 216 and pair of electrodes 224 is dictated by the etch process used to pattern release layer 204. Both factors may hinder the quality and/or applicability of the MEMS resonator.
Thus, a method to form a MEMS structure having a member suspended above a substrate is described herein.